This is an incredibly difficult question to answer, as there are a vast amount of variables involved. As for the cancer tissue in breast cancer patients, the likely explanation for the increase in lactate is the Warburg effect, which is the observation that cancer cells switch to glyocolytic instead of oxidative metabolism. This would cause a buildup of lactate as the end product of this process, and thus higher cancer tissue lactate levels.

As for the plasma, this is where it becomes difficult. These patients may undergo some type of increased kidney or liver function due to other factors dealing with breast cancer, which excretes the lactate at an improved rate. Another hypothesis is the cancer itself is sequestering the lactate from the plasma using it as an electron shuttle to keep the cells in a reductive state. The list could go on and on with hypotheses, but without more data about the sample set and other measurements, they are purely speculative.

~Adam